Optical communication systems which operate in the wavelength range fom 1100 to 1700 nanometers (nm) are of potentially great importance because the dispersion and losses in an optical fiber are typically very low in this wavelength range. Heterojunction devices incorporating binary III-V alloys and solid solutions of these alloys have been found to be particularly useful for this application because their electronic bandgaps occur in this wavelength range and lattice-matched heterojunctions can be obtained by compositional variations. In particular, ternary and quaternary alloys of In, Ga, As and P on an InP substrate have been found to be useful materials for both light-emitters and detectors.
Problems which have affected the performance of avalanche photodetectors using these materials include bulk tunneling currents which occur at electric fields of the order of 1.5.times.10.sup.5 V/cm in ternary and quaternary compounds used for the light-absorptive region, edge breakdown and multiplication of surface leakage currents at the junction periphery. The tunneling has been reduced by locating the P-N junction with its high electric field in a wide bandgap material separate from the light-absorptive region in the narrower bandgap material. This is the so-called SAM (Separated Absorbing and Multiplying) photodetector structure.
Edge breakdown and surface currents have been reduced by the use of a mesa structure in which the width of the detector decreases with increasing distance from the substrate. However, it is desirable to further reduce the electric field distribution at the junction periphery to further limit the surface field and leakage current at the junction periphery.